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Acid–Base Titrations AP Chemistry Laboratory #6 Catalog No. AP9083
Publication No. 10530A
Introd roduction A common question chemists have to answer is how mu ch of something is present in a sample or a product. If the product contains an acid or base, this question is usually answe answe red by a titrati titration. Acid–base titrations can be used to measure the concentration concentration of an acid or base in solution, to calculate the formula formula (molar) mass of an unknown acid or base, and to determine the equilibrium constant of a weak acid ( K a) or of a weak base ( K b).
Concepts • Weak acid
• Equilibrium constant, K a
• Equivalent mass
• Equivalence point
• Titra Titrati tion on curve curve
Backgrou round Tit Titration is a method of vol analyze an volumetric analys analysis—the use of volume measurements to analyze unknown. In acid–base che chemistry, ry, titration is most often used to analyze analy ze the amount of acid or base in a sample or solution. Consider a solution containing an unknown amount of hy droch rochlloric acid. In a titration exp experim riment, a known volume volume of the hy hydroch rochlloric acid solution would be “t “ titrate rated” by slowly owly adding dropwise a standard solution of a strong base such as sodium hy droxi roxide. (A standard rant , , sodium hyd solution is one whose concentration is accurate accurat ely known.) The titran hydroxide in this case, reacts with and consumes consumes the acid via a neutra neutralization reaction (Equation 1). The exact vol vo lume of base needed to react completely with the acid is measured. measure d. This is called the equivalence point of the titration—the point at whi which stoich stoichiometric amounts of the acid and base have combined. HCl( HCl(aq aq)) + NaOH NaOH(a (aq) q)
→
NaCl(aq) + H2O(l)
Equation 1
Knowing the exact concentration and volume added of the titrant gives the number of moles of sodium hyd hydroxi roxide. The latt latter, in turn turn, is rel related by stoichi stoichiometry to the number of moles of hyd hydroch rochlloric acid initially present in the unknown unknown. Indicato ators are usually added to acid–base titrations to detect the equivalence point. The endpoint of the titration is the point at whi wh ich the indicator cha changes color and signals that the equivalence point CHEM-FAX FAX . . .makes .m akes science teaching easier. easier.
IN10530A 072303
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has indeed been reached. For example, in the case of the neutralization reaction shown in Equation 1, the pH of the solution would be acidic (< 7) before the equivalence point and basic (> 7) after the equivalence point. The pH at the equivalence point should be ex actly 7, corresponding to the neutral products (sodium chloride and water). An indicator that changes color around pH 7 is therefore a suitable indicator for the titration of a strong acid with a strong base. The progress of an acid–base titration can also be followed by measuring the pH of the solution being analyzed as a function of the volume of titrant add ed. A plot of the resulting data is called a pH curve or titration curve. Titration curves allow a precise determination of the equivalence point of the titration without the use of an indicator. In this experiment the equivalent mass of an unknown acid will be determined by titration. The equivalent mass is defined as the mass of the acid that supplies one mole of hydrogen ions. The acid, a solid crystalline substance, is weighed out and titrated with a standard solution of sodium hydroxide. From the moles of base used and the mass of the acid, the equivalent mass of the acid is calculated. The acid is then titrated a second time with the standard solution of sodium hydroxide and the course of the titration is followed by using a pH meter. A plot is constructed with pH on the vertical (y) axis and the volume of NaOH on the horizontal (x) axis. From this graph the value of the equilibrium constant ( K a) for the dissociation of the acid is determined. An acid may contain one or more ionizable hydrogen atoms in the molecule. The equivalent mass of an acid is the mass that provides one mole of ionizable hydrogen ions. It can be calculated from the molar mass divided by the number of ionizable hydrogen atoms in a molecule. For example, hydrochloric acid, HCl, contains one ionizable hydrogen atom—the molar mass is 36.45 g/mole, and its equivalent mass is also 36.45 g/mole. Sulfuric acid, H 2SO4, contains 2 ionizable hydrogen atoms—the molar mass of H2SO4 is 98.07 g/mole but its equivalent mass is 49.04 g/mole. Th us, either 36.45 g of HCl or 49.04 g of H 2SO4 would supply one mole of H + ions when dissolved in water. The equivalent mass is determined by titrating an acid with a standard solution of NaOH. Since one mole of NaOH reacts with one mole of hydrogen ion, at the equivalence point the following relation holds: V b
×
M b = moles base = moles H+
grams acid EM a = ————— moles H+ where V b is the volume of base added at the endpoint, M b is the molarity of base, grams acid is the mass of acid used, and EM a is the equivalent mass of the acid. The concentration of the NaOH solution must be accurately known. To “standardize” the NaOH, that is, to find its exact molarity, the NaOH is titrated against a solid acid, potassium hydrogen phthalate (abbreviated KHP). The KHP is chosen because it is easily dried and weighed and has a relatively high equivalent mass. The formula of KHP is: KHP contains one ionizable H+. The titration is followed using phenolphthalein as an indicator.
or
KHC8H4O4
© 2003 Flinn Scientific,Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Acid–Base Titrations,Cata l og No. A P 9 0 8 3,f rom Flinn Scientific, Inc. No part of t his material may be reproduced or transmitted in any form or by any means, electronic or mechanical,including,but not limited to photocopy,recording,or any information storage and retrieval system,without permission in writing from Flinn Scientific,I nc.
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The graph of pH versus volume of NaOH added (see Figure 1) is obtained by carefully following the titration with a pH meter. There is a significant change in pH in the vicinity of the equivalence point. Note that when a weak acid is titrated with a strong base, the equivalence point is NOT at pH 7, but is on the basic side. The value of the equilibrium constant for the dissociation of the acid is obtained from the graph. If the dissociation of the acid is represented as: HA + H2O
→ ←
H3O+ + A–
the equilibrium constant expression is: [H3O+][A–] K a = ————— [HA] When the acid is half neutralized, [HA] = [A–], these terms cancel in the above equation, and K a = [H3O+]. Therefore, when the acid is half-neutralized, the pH = pK a. The point where pH is equal to p K a can be found from the graph. Refer to Figure 1.
Figure 1. pH during titration of a monoprotic weak acid with sodium hydroxide A = Volume NaOH at equivalence point B = 1/2 A = the volume of NaOH required to neutralize one-half the acid when half-neutralized C = pH when the acid is half neutralized = pK a
Experiment Overview The purpose of this experiment is to standardize a sodium hydroxide solution and use the standard solution to titrate an unknown solid acid. The equivalent mass of the solid acid will be determined from the volume of sodium hydroxide added at the equivalence point. The equilibrium constant, K a, of the solid acid will be calculated from the titration curve obtained by plotting the pH of the solution versus the volume of sodium hydroxide added.
© 2003 Flinn Scientific,Inc. All Rights Reserved. Reproduction permission is granted only to science teach ers who have purchased Acid–Base Ti trations,Catal og No . A P 90 83 ,from Flinn Scientifi c, Inc. No part of this material may be reproduced or transmitted in any form or by any means,e lectronic or mechanical,including,but not limited to photocopy,re cording,or any information storage and retri eval system,without permission in writing from Flinn Scientific,Inc.
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Pre-Lab Questions 1. Calculate the equivalent mass of each of the following acids. a. HC2H3O2
b. KHCO3
c. H2SO3
2. Calculate the molarity of a solution of sodium hydroxide, NaOH, if 23.64 mL of this solution is needed to neutralize 0.5632 g of potassium hydrogen phthalate.
3. It is found that 24.68 mL of 0.1165 M NaOH is needed to titrate 0.2931 g of an unknown acid to the phenolphthalein end point. Calculate the equivalent mass of the acid.
4. The following data was collected for the titration of 0.145 g of a weak acid with 0.100 M NaOH as the titrant: Volume NaOH added, mL 0.00 5.00 10.00 12.50 15.00 20.00 24.00 24.90 25.00 26.00 30.00
pH 2.88 4.15 4.58 4.76 4.93 5.36 6.14 7.15 8.73 11.29 11.96
© 2003 Flinn Scientific,Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Acid–Base Ti trations,Catal og N o. A P 90 83 ,from Flinn Scient ific, Inc. No part of this material may be reproduced or transmitted in any form or by any means, electronic or mechanical,including,but not limited to photocopy,recording,or any information storage and retrieval system,without permission in writing from Flinn Scientific,I nc.
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a. Graph the data on the chart below.
b. What is the pH at the equivalence point?
c.
Give the K a and pK a value of the acid. Explain.
d. The following acid–base indicators are available to follow the titration. Which of them would be most appropriate for signaling the endpoint of the titration? Explain.
Indicator Bromphenol blue Bromthymol blue Thymol blue
Color Change Acid Form Base Form yellow blue yellow blue yellow blue
pH Transition Interval 3.0–5.0 6.0–7.6 8.0–9.6
© 2003 Flinn Scientific,I nc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Acid–Base Titrations,Catal og No. A P 90 83 ,from Flinn Scient ific, Inc. No part of this material may be reproduced or transmitted in any form or by any means, electronic or mecha nical,including,but not limited to photocopy,recording,or any information storage and retrieval system,without permission in writing from Flinn Scientific,I nc.
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Materials Buffer solution, pH 7, 50 mL
Balance, (0.001- or 0.0001-g precision)
Sodium hydroxide solution, NaOH, 0.1 M, 150 mL
pH sensor or pH meter
Phenolphthalein indicator solution, 1.0%, 1 mL
Buret, 50-mL
Potassium hydrogen phthalate, KHC8H4O4, 2 g
Weighing dish
Unknown weak acid, 1.5 g
Desiccator
Ring stand and buret clamp
Erlenmeyer flask, 125-mL or 250-mL
Beaker, 250-mL
Wash bottle
Funnel
Magnetic stirrer and stir bar
Safety Precautions Dilute sodium hydroxide solutions are irritating to skin and eyes. Phenolphthalein is an alcoholbased solution and is flammable. It is moderately toxic by ingestion. Keep away from flames and other ignition sources. Avoid contact of all chemicals with eyes and skin and wash hands thoro ughly with soap and water before leaving the laboratory. Wear chemical splash goggles and chemicalresistant gloves and apron.
Procedure Part A. Standardization of a Sodium Hydroxide Solution 1. Obtain a sample of potassium hydrogen phthalate (KHP) that has been previously dried in an oven and stored in a desiccator. 2. On an analytical balance, accurately weigh 0.4 to 0.6 grams of KHP in a previously tared weighing dish. Record the mass of the KHP in the Standardization Data Table. 3. Transfer the KHP into an Erlenmeyer flask—pour the solid through a funnel into the flask. Use water from a wash bottle to rinse all of the remaining solid in the weighing dish or in the funnel into the flask as well. 4. Add about 40 mL of distilled water to the flask and swirl until all the KHP is dissolved. 5. Obtain about 75 mL of the sodium hydroxide, NaOH, solution. 6. Clean a 50-mL buret, then rinse it with three small portions (about 7 mL each) of the NaOH solution. 7. Fill the buret to above the zero mark with the NaOH solution. 8. Open the buret stopcock to allow any air bubbles to escape from the tip. Close the stopcock when the liquid level is between the 0- and 10-mL mark s.
© 2003 Flinn Scientific,I nc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Acid–Base Ti trations,Catalog No. A P 90 83 ,from Flinn Scient ific, Inc. No part of this material may be reproduced or transmitted in any form or by any means, electronic or mechanical,including,but not limited to photocopy,recording,or any inf ormation storage and retrieval system,without permission in writing from Flinn Scientific,I nc.
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9. Measure the precise volume of the solution in the buret and record this value in the Standardization Data Table as the “initial volume.” Note: Volumes are read from the top down in a buret. Always read from the bottom of the meniscus, remembering to include the appropriate number of significant figures. (See Figure 2.) 10. Position the buret over the Erlenmeyer flask so that the tip of the buret is within the flask but at least 2 cm above the liquid surface. 11. Add three drops of phenolphthalein solution to the KHP solution in the flask.
Figure 2. How to read a buret volume.
12. Begin the titration by adding 1.0 mL of the NaOH solution to the Erlenmeyer flask, then closing the buret stopcock and swirling the flask. 13. Repeat step 12 until 15 mL of the NaOH solution have been added to the flask. Be sure to continuously swirl the flask. 14. Reduce the incremental volumes of NaOH solution to 1 ⁄ 2 mL until the pink color starts to persist. Reduce the rate of addition of NaOH solution to drop by drop until the pink color persists for 15 seconds. Remember to constantly swirl the flask and to rinse the walls of the flask with distilled water before the endpoint is reached. 15. Measure the volume of NaOH remaining in the buret, estimating to the nearest 0.01 mL. Record this value as the “final volume” in the Standardization Data Table. 16. Repeat the standardization titration two more times. Rinse the Erlenmeyer flask thoroughly between trials with deionized water.
Part B. Determination of the Equivalent Mass of an Unknown Acid 1. Accurately weigh about 0.3–0.4 g of a sample of the unknown acid in a weighing dish using an analytical balance. Record the mass in the Equivalent Mass Data Tabl e. 2. Dissolve the unknown acid in 40 mL of distilled water and titrate to the phenolphthalein end point as above in steps 5 through 16. 3. Record the initial and final volumes of NaOH solution in the Equivalent Mass Data Tabl e. 4. Repeat one more time. Choose a mass for the second sample so that the volume of NaOH needed is about 45 mL if using a 50-mL buret, or about 22 mL if using a 25-mL buret.
Part C. Determination of the pK a of the Unknown Acid 1. Set up a pH meter and electrode. Calibrate the pH meter using a buffer solution of pH 7.00. Rinse the electrode well with distilled water. 2. On the analytical balance, weigh a sample of the unknown acid that requires approximately 20 mL of titrant. 3. Dissolve the sample in approximately 100 mL distilled water in a 250-mL beaker. © 2003 Flinn Scientific,I nc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Acid–Base Titrations,Catal og No. A P9 08 3,f rom F linn Scient ific, Inc. No part of this material may be reproduced or transmitted in any form or by any means, electronic or mecha nical,including,but not limited to photocopy,recording,or any information storage and retrieval system,without permission in writing from Flinn Scientific,I nc.
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4. Fill the buret with the now standardized NaOH solution. Record the initital volume as the “initial buret reading” in the pK a Data Table 5. Set the beaker containing the unknown acid solution on a magnetic stirrer. Clamp the pH electrode so it is submerged in the acid solution (Figure 3). Be sure the stir bar does not hit the electrode. Set the stir bar gently spinning. 5. When the pH reading has stabilized, record the initial pH of the solution in p K a Data Table. 6. Add about 1 mL of sodium hy droxide solution to the beaker. Record the exact buret reading in pK a Data Table.
Figure 3. Setup.
7. Record the pH of the solution next to the buret reading in the pK a Data Table. 8. Add another 1-mL increment of sodium hydroxide solution. Record both the buret reading and the pH in pK a Data Table. 9. Continue adding sodium hydroxide in 1-mL portions. Record both the bu ret reading and the pH after each addition. 10. When the pH begins to increase by more than 0.3 pH units after an addition, decrease the portions of sodium hydroxide added to about 0.2 mL. 11. Continue adding sodium hydroxide in about 0.2 mL increments. Record both the buret reading and the pH after each addition. 12. When the pH change is again about 0.3 pH units, resume adding the sodium hydroxide in 1-mL increments. Continue to record both the buret reading and the pH after each addition. 13. Stop the titration when the pH of the solution is greater than 12. Record the final volume of solution in the buret and the final pH. 14. Graph the data, with pH on the vertical axis and volume NaOH on the horizontal axis. Make the graph large enough to reflect the care taken with the pH and volume measurements.
Disposal Dispose of the titrated solutions, the sodium hydroxide solution, and any solid acid as directed by your instructor.
© 2003 Flinn Scientific,Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Acid–Base Titratio n s,C at al og No. A P 9 08 3 ,from Flinn Scienti fic, Inc. No part of this material may be reproduced or transmitted in any form or by any means, electronic or mechanical,includi ng,but not limited to photocopy,recording,or any information storage and retrieval system,without permission in writing from Flinn Scientifi c,Inc.
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Data Tables Standardization Data Table Trial 1
Trial 2
Trial 3
Mass KHP, g Final Volume, mL Initial Volume, mL Volume of NaOH added, mL Molarity NaOH (Average) ______ M
Equivalent Mass Data Table Trial 1
Trial 2
Mass Acid, g Final Volume, mL Initial Volume, mL Volume of NaOH added, mL
Equivalent Mass (Average) _________ g/mol
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pK a Data Table Mass of Unknown Acid Standard NaOH Concentration Initial Buret Reading Initial pH Buret Reading (mL)
pH
Buret Reading (Con’t.)
pH
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Post-Lab Calculations and Questions (Use a separate sheet of paper to answer the following questions.)
1. From the standardization data, calculate the molarity of the sodium hydroxide solution for each trial. Average the values and enter the average in the Standardization Data Table. 2. From the equivalent mass data, calculate the equivalent mass of the unknown acid for each trial. Average the values and enter the average in the Equivalent Mass Data Table. 3. Why is equivalent mass determined and not molar mass? 4. Why must the KHP and the acid samples be dried? If they are not dried, how would the results change (high or low)? 5. Why must NaOH be standardized? Why can’t an exact solution of NaOH be prepared? 6. From the graph of pH versus volume of NaOH, determine the pK a of the unknown acid. Convert this value to K a. 7. Why is the equivalence point in the titration of the unknown acid with sodium hydroxide not at pH 7?
© 2003 Flinn Scientific,I nc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Acid–Base Titrations,Catalog No. A P 90 83 ,from Flinn Scientifi c, Inc. No part of this material may be reproduced or transmitted in any form or by any means, electronic or me chanical,including,but not limited to photocopy,recording,or any information storage and retrieval system,without permission in writing from Flinn Scientific,Inc.
